(a) Field of the Invention
The present invention relates to a contact color image sensor using an amorphous semiconductor film.
(b) Description of the Prior Art
Generally, when an image on a document is read out by, e.g., a facsimile system, the image on the document is reduced in size by a lens system and a corresponding image is formed on a photoelectric converter consisting of, e.g., a CCD chip having 2,048 bits and a length of 32 to 40 mm. In such an image sensor, for example, 210 mm which is the width of an ISO A4 size document must be reduced from 210 to 32 mm. In this case, a long optical path from the document surface to the photoelectric converter prevents the provision of a compact device.
In view of this problem, so-called contact image sensors have been developed wherein a photoelectric converter having the same array length as a document width is used, that is, an image on a document is formed on the photoelectric converter at the same magnification, i.e., 1:1. In such a contact image sensors, since a same magnification image forming system such as a rod lens array can be used as an image forming system, an optical path from the document surface to the photoelectric converter is sufficiently shortened to between 10 and 20 millimeters.
An image sensor which can read out a color image for providing a color copying machine has also been developed. In a conventional color image sensor, color separation of input image is performed in a main scanning direction. For this reason, the number of pixels in a main scanning direction is equivalently increased to 3 or 4 times of the number of pixels of a monotone contact image sensor, and a photosensitive area for one pixel is reduced, thereby degrading S/N ratios of output signals. Therefore, a method has been suggested in which the color separation is performed in a subscanning direction, as in "OPTICAL ENGINEERING" November/December 1981 Vol. 20, No. 6. According to this method, the number of pixels in the main scanning direction is the same as that of a monotone image sensor, thereby obtaining high S/N ratios of output signals.
Although a CCD chip is used as a photoelectric converter in the above-mentioned reference, such a CCD chip cannot have a length which is as long as a document width as is described above. Then, a contact color image sensor by the method shown in the above-mentioned reference cannot be formed.
A contact image sensor having a photoelectric converter formed of an amorphous semiconductor film which can be formed in a large area is currently receiving a lot of attention. In this image sensor using amorphous semiconductor film, so-called pixel electrodes are separately formed on a substrate so as to correspond to pixels of a document in one-to-one correspondence. Then, an amorphous semiconductor film is formed on the electrode and a common transparent electrode is formed thereon. The amorphous semiconductor film generates electric charge corresponding to the quantity of light of the input light and stores them. The stored electric charges are read out through the pixel electrode and the common transparent electrode, thereby obtaining an output signals.
When a contact image sensor using an amorphous semiconductor film is realized by adopting a method shown in the above-mentioned reference, three or four rows of pixel electrodes corresponding to three or four colors are arranged and, in correspondence with these electrodes, color filters are provided on a common transparent electrode. However, in this case, the precision of color separation is important. In other words, the pixel electrodes are formed on a single substrate together with connecting electrodes for connecting the pixel electrodes to an external circuit. However, in these connecting electrodes, the connecting electrodes from an inner one or two rows of the pixel electrodes must be formed to extend between two adjacent pixel electrodes of each of the outer rows. In this manner, in a structure in which the connecting electrodes extend between the pixel electrodes, the light which passes or does not pass through a color filter can be irradiated onto the portions of the amorphous semiconductor film formed on the connecting electrodes. In such an image sensor, only the portions of an amorphous semiconductor film on the pixel electrodes are used as photoelectric converting regions. Then, when the light is irradiated on the portions of the amorphous semiconductor film on the connecting electrodes as described above, it results in noise. In addition, particularly in the color image sensor, when the light of undesirable colors or the light which does not pass through the color filter is irradiated onto the portions of the amorphous semiconductor film formed on the connecting electrodes, the color reproduction property is considerably degraded.